if you think about how the springfield musket tended to shoot high because of its high trajectory, it seems to me that a very high number of casualties would have been inflicted on the confederates when they climbed 4 feet into the air. Maybe at that point in the charge the confederates were just happy to have that fence there to stop some of the fire coming at them. I do believe unsolved history had it right that a large portion of the confederates took refuge there, if they had taken down the fence a larger portion would have made the breach in the federal lines